Confused about how many collision domains in this internetwork

[Susan]

Hello Todd~
I was confuesed about the exaple that you give in the 6th CCNA study guide about the collision domain.

See the attached .
In this book ,you say,

"There are nine collsion domains.
Q1:The all-hub network is one collision domain.
(Why the other two hubs lay at the botton doesnot creat 2 collsion domains? or there could be 3 collisions? each hub has one.)

Q2: The bridge network equals three collision domain.
(why? The bridge is just like switch ,and it only has two hubs. One hub creat one collsion domain. why there are 3 collision domains?)



Q3:And in the switch network of five collsion domains--one for each switch port.
(why? aren't there only two switches? And each switch only has one host ,there should be 2 collision domain.)

And you've got a total of nine."
(I found 7 different collision domains.)
________
Toyota celica (http://www.toyota-wiki.com/wiki/Toyota_Celica)

[lildeezul]

Q1- IN the all hub network, you must know that HUBS DO NOT break up collision domains. hubs are dumb.. They only replicate data. and collision domain is a domain where if two packet are sent at the same time and collide, then the whole area (or collision domain ) will be affected.
Hubs do not break up colliision domains.. So if there is a network with 55 hubs, all 55 hubs belong to 1 collision domain, becuase they do not break up collision domains.



Q3-remember with bridges and switches BREAK UP COLLISION DOMAINS.. each port of the switch/bridge is in its own collision domains. So when in the picture with all the switches there are five collision domains, becuase each port of the switch is its own collision domain. so the top switch has two connections to the other two switches, Thats two collision domains, then each bottom switch has a connection to a pc. that another 2 collision domain. and then the top switch is connected to the router, thats 1 collision domain, so all in all it is a total of 5 collision domains. Remember each switchport belongs it its own collision domain.

Q2- Back to the same concept of question 1.. hubs do not break up collision domains. they do nothing with the collision or broadcast domain. so no matter how much hubs you have your only going to have a x amount of collision domains. Hubs do not increment the number of collision domain.
so each port of the brige is its own collision domain. So the brigde has to connection to the hubs, that means two collision domains, becuase each port of the bridge/switch is its own collision domain. remember hubs do not change the number of collision domains. Therefore the number of hubs doesnt matter. 30 hubs could be connected in daisy chain to the other hubs, and it will still be 2 collision domains.




Hope this helps.

[Susan]

To: lildeezul
Thank a lot.
I do appreciate your help~`

And one more question here.
How many broadcast domains in this internetwork(see the attached)? One or two?
________
PLYMOUTH SUPERBIRD SPECIFICATIONS (http://www.dodge-wiki.com/wiki/Plymouth_Superbird)

[Bravo020]

From what I can see in the image, 2 broadcast domains exist.

If the Router R1 was replaced with a Switch, then only a single broadcast domain would exist. But since a router breaks up broadcast domains by default then the answer is 2.

Switches on the other hand as discussed already break up collision domains, but not broadcast domains (unless of course VLANs are used).

The hubs break up nothing, as they just send the signal that they receive out of all available ports.

Hope that this helps.
Regards.

[lildeezul]

good post bravo

[Susan]

Got it.
Thanks a lot.
________
Subaru r1 (http://www.toyota-wiki.com/wiki/Subaru_R1)

[iont]

Hello,

i would like to ask if at the second network, we had only the router, without the switches, then we would have two broadcast domains and zero collision domains ? i am not certain about the collision domains .

we would have zero or two collision domains ?

Thank you .